Method for hydrogenating an anthraquinone compound

ABSTRACT

A process for hydrogenating an anthraquinone compound or a mixture of two or more thereof utilizes specific catalysts comprising, as active metal, a metal of transition group VIII of the Periodic Table of the Elements.

The present invention relates to a process for hydrogenating ananthraquinone compound or a mixture of two or more thereof by contactingthe anthraquinone compound or the mixture of two or more thereof with acatalyst comprising, as active metal, at least one metal of transitiongroup VIII of the Periodic Table of the Elements and a process forpreparing hydrogen peroxide by the anthraquinone process comprising ahydrogenation step as defined above and the reaction of theanthraquinone compound obtained in this step with an oxygen-containinggas.

Virtually all of the hydrogen peroxide produced worldwide is prepared bythe anthraquinone process.

The process is based on the catalytic hydrogenation of an anthraquinonecompound to give the corresponding anthrahydroquinone compound followedby reacting the latter with oxygen to give hydrogen peroxide andsubsequently removing the hydrogen peroxide formed by extraction. Thecatalyst cycle is closed by rehydrogenation of the reformedanthraquinone compound.

The basic reactions are summarized in the scheme below:

The anthraquinone compounds used are typically dissolved in a mixture ofseveral organic solvents. The resulting solution is referred to as theworking solution. In the anthraquinone process, this working solution isusually passed through the above-described process steps in a continuousmanner.

The anthraquinone process is reviewed in Ullmann's Encyclopedia ofIndustrial Chemistry, 5th ed., vol. A13, pp. 447-456.

A particularly important step of the anthraquinone process is thehydrogenation step, in which the anthraquinone compound in the workingsolution is hydrogenated in the presence of a catalyst to give thecorresponding anthrahydroquinone compound.

Said catalytic hydrogenation can be carried out in suspension or in afixed bed in various reactor types. The relevant prior art is describedin detail in EP-A-0 672 617, for example. This reference relates to aprocess of the subject type using a fixed-bed reactor comprising acatalyst bed having an open structure. It is suggested to use palladiumon a support, such as activated carbon, aluminum oxide or silica gel, asa catalyst.

EP-A-0 102 934 describes another version of the anthraquinone processwhich likewise utilizes a fixed bed having a structure containingspecific, parallel passages. According to this reference, useful activemetals for the catalysts described therein include noble metals, eg.palladium, platinum, rhodium or mixtures thereof.

U.S. Pat. No. 4,428,923 describes an anthraquinone process which iscarried out in suspension and utilizes a loop reactor and palladiumblack as a catalyst.

EP-A-0 778 085 and WO 96/18574 describe the use of Pd, Rh, Pt or Ru asactive metals in a catalyst suitable for the anthraquinone process,where conventional materials, such as Al₂O₃ or SiO₂, are used as supportmaterials for the catalysts described.

The prior art catalysts did not always meet the requirements for suchcatalysts, such as a high activity together with a high selectivity.Furthermore, it was not always possible to achieve sufficiently highspace-time yields.

It is an object of the present invention to provide novel processes forhydrogenating an anthraquinone compound using catalysts previously notused for said hydrogenation.

We have found that this object is achieved, in one embodiment, by aprocess for hydrogenating an anthraquinone compound or a mixture of twoor more thereof by contacting the anthraquinone compound or the mixtureof two or more thereof with a catalyst to obtain an anthrahydroquinonecompound or a mixture of two or more thereof, which comprises using acatalyst (catalyst 1) comprising at least one homogeneous compound of atleast one metal of transition group VIII of the Periodic Table of theElements alone or together with at least one metal of transition group Ior VII of the Periodic Table of the Elements, deposited on a support insitu.

The present invention further provides a process for hydrogenating ananthraquinone compound as described above, which comprises using acatalyst (catalyst 2) comprising, as active metal, at least one metal oftransition group VIII of the Periodic Table of the Elements, alone ortogether with at least one metal of transition group I or VII of thePeriodic Table of the Elements, applied to a support, the support havinga mean pore diameter of at least 50 nm and a BET surface area of at most30 m²/g and the amount of active metal being from 0.01 to 30% by weight,based on the total weight of the catalyst, and the ratio of the surfaceareas of the active metal and the catalyst support preferablybeing<0.05.

The invention further provides a process for hydrogenating ananthraquinone compound as defined above, which comprises using acatalyst (catalyst 3) comprising, as active metal, at least one metal oftransition group VIII of the Periodic Table of the Elements, alone ortogether with at least one metal of transition group I or VII of thePeriodic Table of the Elements, in an amount of from 0.01 to 30% byweight, based on the total weight of the catalyst, applied to a support,from 10 to 50% of the pore volume of the support being macropores havinga pore diameter of from 50 nm to 10,000 nm and from 50 to 90% of thepore volume of the support being mesopores having a pore diameter offrom 2 to 50 nm, the sum of the pore volumes being 100%.

In another embodiment, the present invention provides a process forhydrogenating an anthraquinone compound as described above, whichcomprises using a catal)-st (catalyst 4) comprising, as active metal, atleast one metal of transition group VIII of the Periodic Table of theElements with at least one metal of transition group I or VII of thePeriodic Table of the Elements, in an amount of from 0.01 to 30% byweight, preferably from 0.2 to 15% by weight, based on the total weightof the catalyst, applied to a support, the support having a mean porediameter of at least 0.1 ^(μ)m, preferably at least 0.5 ^(μ)m, and asurface area of at most 15 m²/g, preferably at most 10 m²/g.

The invention further provides a process for hydrogenating ananthraquinone compound as described above, which comprises using as acatalyst (catalyst 5) a monolithic supported catalyst obtainable bysequentially heating in air and cooling down a support material in theform of a metal fabric or metal foil, followed by coating with an activecomponent under reduced pressure, subsequent cutting and shaping of thecoated support material and finally processing to give a monolithicsupported catalyst, using, as active metal, at least one metal oftransition group VIII of the Periodic Table of the Elements alone ortogether with at least one metal of transition group I or VII of thePeriodic Table of the Elements.

Any metal of transition group VIII of the Periodic Table of the Elementscan in principle be used as active metal. Platinum, rhodium, palladium,cobalt, nickel or ruthenium or a mixture of two or more thereof arepreferably used as active metals, in particular ruthenium. It is inprinciple likewise possible to use any metal of transition group Iand/or VII, preference being given to using copper and/or rhenium.

For the purposes of the present invention, the terms “macropores” and“micropores” are used as defined in Pure Appl. Chem. 45 (1976) p. 79,namely to describe pores whose diameter is above 50 nm (macropores) orfrom 2 nm to 50 nm (mesopores).

The active metal content is generally from about 0.01 to about 30% byweight, preferably from about 0.01 to about 5% by weight, especiallyfrom about 0.1 to about 5% by weight, in each case based on the totalweight of the catalyst used, the preferred contents of the catalysts 1to 5 being specifically reported in the discussion of these catalysts.

“Anthraquinone compound” encompasses in principle all anthraquinonecompounds and the corresponding tetrahydroanthraquinone compoundssuitable for use in the preparation of hydrogen peroxide. The preferredcompounds which can be used are described briefly in the chapter“Processing Procedure” below.

The catalysts 1 to 5 defined above will now be described in detail byway of example with reference to the use of ruthenium as active metal.The details given below also apply to the other active metals which canbe used as defined herein.

CATALYST 1

The process of the invention can be carried out in the presence of acatalyst 1 comprising at least one homogeneous compound of at least onemetal of transition group VIII of the Periodic Table of the Elementsdeposited on a support in situ, with or without at least one compound ofat least one metal of transition group I or VII of the Periodic Table ofthe Elements. To prepare the catalysts, a homogeneous metal compound isco-fed into the reactor during the reaction together with the feed to bedeposited on a support present in the reactor during the reaction.

It is also possible to introduce the homogeneous metal compound into thereactor prior to the reaction to be deposited on a support present inthe reactor during a hydrogen treatment.

For the purposes of the present invention, “in situ” means that thecatalyst is not prepared and dried separately and then fed into thereactor as a ready-to-use catalyst, but, according to the presentinvention, is formed in the reactor immediately before or during theactual hydrogenation.

For the purposes of the present invention, “homogeneous compound of ametal of transition group VIII, I or VII of the Periodic Table of theElements” or “homogeneous ruthenium compound” means that the metalcompound used according to the invention is soluble in the surroundingmedium, ie. in the anthraquinone compound employed which is yet to behydrogenated or in a mixture of these compounds with at least onesolvent.

Useful metal compounds are in particular nitrosyl nitrates and nitrates,but also halides, carbonates, carboxylates, acetylacetonates, chlorocomplexes, nitrido complexes and amine complexes and also oxide hydratesor mixtures thereof. Preference is given to ruthenium nitrosyl nitrate,ruthenium(III) chloride, ruthenium(III) nitrate and ruthenium oxidehydrate.

Although there are no particular restrictions as to the amount of themetal compound applied to the carrier(s) in the process of theinvention, in view of sufficient catalyst activity and process economy,the metal salt or metal complex is applied to the carrier in an amountsufficient to deposit from 0.01 to 30% by weight, based on the totalweight of the catalyst, of active metal on the carrier(s). Said amountis more preferably from 0.2 to 15% by weight, particularly preferablyabout 0.5% by weight.

The supports present in the reactor are preferably metal meshes, metalrings and steatite bodies as described, among others, in EP-A-0 564 830and EP-A-0 198 435. The supports particularly preferably used in thepresent invention and their preparation will nevertheless be describedbriefly below.

Particular preference is given to using metallic support materials, suchas the stainless steels having the material numbers 1.4767, 1.4401,2.4610, 1.4765, 1.4847, 1.4301, etc. since they can be surface-roughenedby heat treatment before they are coated with active components.Particular preference is given to using Kanthal (material number 1.4767)or aluminum-containing metals as mesh materials. Kanthal is an alloycontaining about 75% by weight Fe, about 20% by weight Cr and about 5%by weight A1. Heat treatment is effected by heating the metallicsupports cited above in air at from 600 to 1100° C., preferably at from800 to 1000° C., for from 1 to 20 hours, preferably for from 1 to 10hours, and recooling. This pretreatment is crucial for the activity ofthe catalyst since it is virtually impossible to deposit ruthenium insitu on the metallic carriers without this heat treatment. After thistreatment at elevated temperature, the support is coated with theruthenium compound.

In a further preferred embodiment, the carriers described above may becoated by vapor deposition with a layer of a palladium metal, such asNi, Pd, Pt, Rh, preferably Pd, having a thickness of from about 0.5 toabout 10 nm, especially about 5 nm, as is likewise described in theabovementioned EP-A-0 564 830.

As can be seen from the examples according to the invention, aparticular catalyst used as a support in the present invention is a meshof heat-treated Kanthal onto which a Pd layer having a thickness ofabout 5 nm has been vapor-deposited to facilitate the deposition of theactive metal.

It is, however, also possible to use conventional catalyst supportsystems, such as activated carbon, silicon carbide, aluminum oxide,silicon dioxide, titanium dioxide, zirconium dioxide, magnesium oxide,zinc oxide or mixtures thereof, in each case in the form of spheres,extrudates or rings. Among these, particular preference is given toaluminum oxide and zirconium dioxide. The pore size and the poredistribution are completely uncritical. It is possible to use bimodalsupports or also any other type of support. The supports are preferablymacroporous.

Catalyst 1 and its preparation are described in more detail in DE-A 19622 705.4, the relevant contents of which are fully incorporated hereinby reference.

CATALYST 2

The catalysts 2 used according to the present invention can be preparedindustrially by applying at least one metal of transition group VIII ofthe Periodic Table of the Elements and, if desired, at least one metalof transition group I or VII of the Periodic Table of the Elements to asuitable support.

The application can be achieved by impregnating the support with aqueousmetal salt solutions such as aqueous ruthenium salt solutions, byspraying appropriate metal salt solutions onto the support or by othersuitable methods. Suitable metal salts of transition groups I, VII andVIII of the Periodic Table of the Elements are the nitrates, nitrosylnitrates, halides, carbonates, carboxylates, acetylacetonates, chlorocomplexes, nitrito complexes or amine complexes of the correspondingmetals, preference being given to the nitrates and nitrosyl nitrates.

In the case of catalysts comprising not only a metal of transition groupVIII of the Periodic Table of the Elements but also further metals asactive metal on the support, the metal salts or metal salt solutions canbe applied simultaneously or in succession.

The supports which have been coated or impregnated with the metal saltsolution are then dried, preferably at from 100° C. to 150° C., and ifdesired calcined at from 200° C. to 600° C., preferably at from 350° C.to 450° C. In the case of separate impregnations, the catalyst is driedand if desired calcined, as described above, after each impregnationstep. The order in which the active components are applied can beselected without restriction.

The coated, dried and if desired calcined supports are subsequentlyactivated by treatment in a gas stream comprising free hydrogen at fromabout 30° C. to about 600° C., preferably from about 150° C. to about450° C. The gas stream preferably comprises from 50 to 100% by volume ofH₂ and from 0 to 50% by volume of N₂.

The metal salt solution(s) are applied to the support or supports insuch an amount that the total active metal content, in each case basedon the total weight of the catalyst, is from about 0.01 to about 30% byweight, preferably from about 0.01 to about 5% by weight, morepreferably from about 0.01 to about 1% by weight, in particular fromabout 0.05 to about 1% by weight.

The total metal surface area on the catalyst is preferably from about0.01 to about 10 m²/g of catalyst, more preferably from about 0.05 toabout 5 m²/g, in particular from about 0.05 to about 3 m²/g. The metalsurface area is determined by means of the chemisorption methoddescribed by J. LeMaitre et al. in “Characterization of HeterogenousCatalysts”, eds. Francis Delanney, Marcel Dekker, New York 1984, pp.310-324.

In the catalyst used according to the present invention, the ratio ofthe surface areas of the active metal/metals and the catalyst support ispreferably less than about 0.05, the lower limit being about 0.0005.

The support materials which can be used for preparing the catalysts usedaccording to the present invention are those which are macroporous andhave a mean pore diameter of at least about 50 nm, preferably at leastabout 100 nm, in particular at least about 500 nm, and a BET surfacearea of at most about 30 m²/g, preferably at most about 15 m²/g, morepreferably at most about 10 m²/g, in particular at most about 5 m²/g,even more preferably at most about 3 m²/g. More precisely, the mean porediameter of the support is preferably from about 100 nm to about 200^(μ)m, more preferably from about 500 nm to about 50 ^(μ). The surfacearea of the support is preferably from about 0.2 to about 15 m²/g, morepreferably from about 0.5 to about 10 m²/g, in particular from about 0.5to about 5 m²/g, even more preferably from about 0.5 to about 3 m²/g.

The surface area of the support is determined by the BET method by N₂adsorption, in particular in accordance with DIN 66131. The mean porediameter and the pore size distribution are determined by Hgporosimetry, in particular in accordance with DIN 66133.

The pore size distribution of the support is preferably approximatelybimodal, the bimodal pore diameter distribution having maxima at about600 nm and about 20 ^(μ)m representing a specific embodiment of theinvention.

Further preference is given to a support having a surface area of 1.75 m²/g and this bimodal pore diameter distribution. The pore volume of thispreferred support is preferably about 0.53 ml/g.

Macroporous support materials which can be used are, for example,activated carbon, silicon carbide, aluminum oxide, silicon dioxide,titanium dioxide, zirconium dioxide, magnesium oxide, zinc oxide ormixtures of two or more of these, preference being given to usingaluminum oxide and zirconium dioxide.

Catalyst 2 and its preparation are described in more detail in DE-A 19624 484.6, the relevant contents of which are fully incorporated hereinby reference.

CATALYST 3

The catalysts 3 used according to the present invention can be preparedindustrially by applying an active metal of transition group VIII of thePeriodic Table of the Elements, preferably ruthenium or palladium, and,if desired, at least one metal of transition group I or VII of thePeriodic Table of the Elements to a suitable support. The applicationcan be achieved by impregnating the support with aqueous metal saltsolutions, such as ruthenium or palladium salt solutions, by sprayingappropriate metal salt solutions onto the support or by other suitablemethods. Suitable metal salts for preparing the metal salt solutions arethe nitrates, nitrosyl nitrates, halides, carbonates, carboxylates,acetylacetonates, chloro complexes, nitrito complexes or amine complexesof the corresponding metals, preference being given to the nitrates andnitrosyl nitrates.

In the case of catalysts comprising more than one active metal on thesupport, the metal salts or metal salt solutions can be appliedsimultaneously or in succession.

The supports which have been coated or impregnated with the metal saltsolution are then dried, preferably at from 100° C. to 150° C., and ifdesired calcined at from 200° C. to 600° C., preferably at from 350° C.to 450° C. The coated supports are subsequently activated by treatmentin a gas stream comprising free hydrogen at from 30° C. to 600° C.,preferably from 100° C. to 450° C. and in particular from 100° C. to300° C. The gas stream preferably comprises from 50 to 100% by volume ofH₂ and from 0 to 50% by volume of N₂.

In the case of applying more than one active metal to the support andsuccessive application, the support may be dried at from 100° C. to 150°C. and if desired calcined at from 200° C. to 600° C. after eachapplication or impregnation step. The order in which the metal saltsolutions are applied or impregnated can be selected withoutrestriction.

The metal salt solution is applied to the support or supports in such anamount that the active metal content, in each case based on the totalweight of the catalyst, is from 0.01 to 30% by weight, preferably from0.01 to 10% by weight, further preferred from 0.01 to 5% by weight, inparticular from 0.3 to 1% by weight.

The total metal surface area on the catalyst is preferably from 0.01 to10 m²/g of catalyst, particularly preferably from about 0.05 to about 5m²g, more preferably from about 0.05 to about 3 m²/g. The metal surfacearea was determined by means of the chemisorption method described by J.LeMaitre et al. in “Characterization of Heterogenous Catalysts”, eds.Francis Delanney, Marcel Dekker, New York 1984, pp. 310-324.

In the catalyst used according to the present invention, the ratio ofthe surface areas of the at least one active metal and the catalystsupport is less than about 0.3, preferably less than about 0.1, inparticular about 0.05 or less, the lower limit being about 0.0005.

The support materials which can be used for the preparation of thecatalysts used according to the invention have macropores and-mesopores.

The supports which can be used according to the invention have a poredistribution such that from about 5 to about 50%, preferably from about10 to about 45%, more preferably from about 10 to 30%, in particularfrom 15 to 25%, of the pore volume are macropores having a pore diameterof from about 50 nm to about 10,000 nm and from about 50 to about 95%,preferably from about 55 to about 90%, more preferably from about 70 toabout 90%, in particular from about 75 to about 85%, of the pore volumeare mesopores having a pore diameter of from about 2 to about 50 nm, thesum of the pore volumes being 100% in each case.

The total pore volume of the support used according to the invention isfrom about 0.05 to about 1.5 cm³/g, preferably from about 0.1 to about1.2 cm³/g and in particular about 0.3 to about 1.0 cm³/g. The mean porediameter of the support used according to the invention is about 5 toabout 20 nm, preferably about 8 to about 15 nm and in particular fromabout 9 to about 12 nm.

The surface area of the support is preferably from about 50 to about 500m²/g of the support, more preferably from about 200 to about 350 m²/g,in particular from about 200 to about 250 m²/g.

The surface area of the support is determined by the BET method by N₂adsorption, in particular in accordance with DIN 66131. The mean porediameter and the pore size distribution are determined by Hgporosimetry, in particular in accordance with DIN 66133.

It is in principle possible to use any support material known incatalyst preparation, ie. having the pore size distribution definedabove, but preference is given to activated carbon, silicon carbide,aluminum oxide, silicon dioxide, titanium dioxide, zirconium dioxide,magnesium oxide, zinc oxide or mixtures of two or more of these, furtherpreference being given to using aluminum oxide and zirconium dioxide.

Catalyst 3 and its preparation are described in more detail in DE-A 19624 485.4, the relevant contents of which are fully incorporated hereinby reference.

CATALYST 4

The catalysts 4 used according to the present invention can be preparedindustrially by applying an active metal of transition group VIII of thePeriodic Table of the Elements and, if desired, at least one metal oftransition group I or VII of the Periodic Table of the Elements to asuitable support. The application can be achieved by impregnating thesupport with aqueous metal salt solutions, such as ruthenium saltsolutions, by spraying appropriate metal salt solutions onto the supportor by other suitable methods. Suitable ruthenium salts for preparing theruthenium salt solutions and also metal salts of transition groups I,VII and VIII of the Periodic Table of the Elements are the nitrates,nitrosyl nitrates, halides, carbonates, carboxylates, acetylacetonates,chloro complexes, nitrito complexes or amine complexes of thecorresponding metals, preference being given to the nitrates andnitrosyl nitrates.

In the case of catalysts comprising more than one metal on the support,the metal salts or metal salt solutions can be applied simultaneously orin succession.

The supports which have been coated or impregnated with the rutheniumsalt solution or metal salt solution are then dried, preferably at from100° C. to 150° C., and if desired calcined at from 200° C. to 600° C.

The coated supports are subsequently activated by treatment in a gasstream comprising free hydrogen at from 30° C. to 600° C., preferablyfrom 150° C. to 450° C. The gas stream preferably comprises from 50 to100% by volume of H₂ and from 0 to 50% by volume of N₂.

If not only the active metal of transition group VIII of the PeriodicTable of the Elements but also metals of transition group I or VII areapplied to the support in succession, the support may be dried at from100° C. to 150° C. and if desired calcined at from 200° C. to 600° C.after each application or impregnation step. The order in which themetal salt solutions are applied or impregnated can be selected withoutrestriction.

The metal salt solution is applied to the support or supports in such anamount that the active metal content, in each case based on the totalweight of the catalyst, is from 0.01 to 30% by weight, preferably from0.2 to 15% by weight, particularly preferably about 0.5% by weight.

The total metal surface area on the catalyst is preferably from 0.01 to10 m²/g of catalyst, more preferably from 0.05 to 5 m²/g, in particularfrom 0.05 to 3 m²/g.

The support materials which can be used for preparing the catalysts usedaccording to the present invention are preferably those which aremacroporous and have a mean pore diameter of at least about 0.1 ^(μ)m,preferably at least about 0.5 ^(μ)m, and a surface area of at most 15m²/g, preferably at most 10 m²/g, particularly preferably at most 5m²/g, in particular at most 3 m²/g. The mean pore diameter of thesupport is preferably from 0.1 to 200 ^(μ)m, in particular from 0.5 to50 ^(μ)m. The surface area of the support is preferably from 0.2 to 15m²/g, more preferably from 0.5 to 10 m²/g, in particular from 0.5 to 5m²/g, especially from 0.5 to 3 m²/g.

The surface area of the support is determined by the BET method by N₂adsorption, in particular in accordance with DIN 66131. The mean porediameter and the pore size distribution are determined by Hgporosimetry, in particular in accordance with DIN 66133. The pore sizedistribution of the support may preferably be approximately bimodal, thebimodal pore diameter distribution having maxima at about 0.6 ^(μ)m andabout 20 ^(μ)m representing a specific embodiment of the invention.

Particular preference is given to a support having a surface area ofabout 1.75 m²/g and this bimodal pore diameter distribution. The porevolume of this preferred support is preferably about 0.53 ml/g.

Macroporous support materials which can be used are, for example,activated carbon, silicon carbide, aluminum oxide, silicon dioxide,titanium dioxide, zirconium dioxide, magnesium oxide, zinc oxide ormixtures of two or more of these, preference being given to usingaluminum oxide and zirconium dioxide.

Catalyst 4 and its preparation are described in more detail in DE-A 19604 791.9, the relevant contents of which are fully incorporated hereinby reference.

CATALYST 5

The catalyst 5 used according to the invention can be prepared bysequentially heating in air and cooling down of a support material inthe form of a metal fabric or metal foil, followed by coating with theabove-described active metal or a combination of two or more thereofunder reduced pressure, subsequent cutting and shaping of the coatedsupport material and finally processing to give a monolithic catalystelement. This catalyst and its preparation are described in more detailin EP-A-0 564 830 and U.S. Pat. No. 4,686,202, the relevant contents ofwhich are fully incorporated herein by reference. The essential featuresof the preparation of this catalyst and its preferred embodiments willherein only be discussed briefly. What was said regarding the activemetals used for catalysts 1 to 4 also applies here.

Particularly suitable examples of metallic support materials in the formof metal foils or metal fabrics are stainless steels, for example thosehaving the material numbers 1.4767, 1.4401, 2.4610, 1.4765, 1.4847,1.4301, etc. since they can be surface-roughened by heat treatmentbefore they are coated with active components. To this end, the metallicsupports are heated in air at from 600 to 1100° C., preferably at from800 to 1000° C., for from 1 to 20 hours, preferably for from 1 to 10hours, and then recooled. This pretreatment is crucial for the activityof the catalyst. After this treatment at elevated temperature, thesupport is coated with the active component. To this end, the support iscoated simultaneously or successively, batchwise or continuously, withthe active components under a reduced pressure of from 10⁻³ to 10−5 mbarby means of an evaporation unit, for example electron beam evaporation,or a sputtering unit. This can be followed by heat treatment under aninert gas or air in order to activate the catalyst.

The aim of the preparation of catalyst layers described here is toprepare highly unordered and defective polycrystalline layers orclusters. It is therefore normally not necessary for the vacuumconditions to be particularly good. Furthermore, alternate deposition ofactive components and structural promoters allows the active componentsto be produced in very finely crystalline or cluster-like form.

Here, the catalyst can be built up systematically, for example in avapor deposition unit containing a plurality of different evaporationsources. Thus, for example, it is possible first to apply an oxide layeror, by reactive evaporation, an adhesive layer to the support. Activecomponents and promoters can be prepared on this base layer in aplurality of alternate layers. By admitting a reactive gas into therecipient, promoter layers of oxides or other compounds can be produced.Interim heat treatment can also be carried out.

Due to this method of production of the catalyst fabric or catalystfoils, the active components have such high adhesion that they can becut, shaped and processed to give monolithic catalyst elements.

A very simple monolithic catalyst is obtained if the catalyst fabric orcatalyst foil is shaped by ring gear rolling and flat and corrugatedfabric or foil is rolled up to form a cylindrical monolith havingidentical vertical channels. However, it is also possible to shape anydesired static mixers from this catalyst material, since the adhesion ofthe catalyst layer is sufficiently high.

The monolithic catalyst elements produced in this way, in the form ofmixed elements, are installed in a reactor and charged with the reactionliquid to be reacted.

PROCESSING PROCEDURE

In the process of the invention, the hydrogenation is generally carriedout at from about 20 to 120° C., preferably from about 30 to 80° C.,pressures employed being usually from about 1 to about 20 bar,preferably from about 2 to 10 bar.

The hydrogenation can be carried out with pure hydrogen or ahydrogen-containing gas.

To achieve a very high selectivity of generally >90%, preferably >95%,the hydrogenation is usually allowed to proceed until a conversion ofabout 50 to 70% is reached.

Preferred anthraquinone compounds used according to the invention are2-alkylanthraquinones, such as 2-ethyl-, 2-tert-butyl, 2-amyl-,2-methyl-, 2-butyl-, 2-isopropyl-, 2-sec-butyl-,2-sec-amylanthraquinone, and polyalkylanthraquinones, such as1,3-diethylanthraquinone, 2,3-dimethylanthraquinone,1,4-dimethylanthraquinone, 2,7-dimethylanthraquinone, and thecorresponding tetrahydroanthraquinone compounds and mixtures of two ormore thereof.

Any solvent known in the prior art as a solvent for anthraquinone oranthrahydroquinone compounds may be used. Preference is given tomixtures of two or more solvent components since such solvent mixturesprovide an optimum balance for the different solubility characteristicsof anthraquinone and anthrahydroquinone compounds. Examples includemixtures of methylnaphthalene and nonyl alcohol, methylnaphthalene andtetrabutylurea, polyalkylated benzene and alkylphosphates ormethylnaphthalene, tetrabutylurea and alkylphosphates.

Nor are there any restrictions on the reactors which can be used in theprocess of the invention so that all reactors known from the prior artand suitable for hydrogenations may be used.

The present invention further provides a process for preparing hydrogenperoxide by the anthraquinone process, which comprises the followingsteps (1) and (2):

(1) hydrogenating an anthraquinone compound or a mixture of two or morethereof by a process as defined above to obtain an anthrahydroquinonecompound or a mixture of two or more thereof, and

(2) reacting the anthraquinone compound or the mixture of two or morethereof with an oxygen-containing gas to give a mixture comprisinghydrogen peroxide and the anthraquinone compound or the mixture of twoor more thereof.

The steps (1) and (2) are preferably conducted continuously, morepreferably continuously with recycling of the anthraquinone compoundobtained in step (2) to step (1), the anthraquinone compound beingrecycled after removal of the hydrogen peroxide formed as a constituentof a working solution.

In a further embodiment of the process of the invention, the hydrogenperoxide is extracted in a further step (3) using an aqueous extractant,preference being given to using pure water.

As regards general procedures for conducting the anthraquinone processcomprising the steps (1) to (3) above, reference is made to the priorart mentioned at the beginning.

The examples which follow illustrate the invention.

EXAMPLES Example 1

A mesh of 1 mm mesh diameter made of heat-treated Kanthal onto which aPd layer having a thickness of 5 nm had been vapor-deposited was placedin a 3.5 l autoclave. A Comparative Example indicated that thisPd-coated mesh was not catalytically active. In the first run, theautoclave was charged with 2 l of a 13% strength solution of2-ethylanthraquinone in a 70:30 mixture of Shellsol® and tetrabutylureatogether with 200 mg of ruthenium nitrosyl nitrate.

The batch was then hydrogenated at a hydrogen pressure of 10 bar for 60minutes. The reaction effluent contained no ruthenium.2-Ethylanthraquinone was converted to 2-ethylanthrahydroquinone at aselectivity of 100% (conversion: 72%). In the second run, 2 l of thissolution were converted with hydrogen over the mesh catalyst withoutadded ruthenium in the same manner.

The reaction effluent contained no traces of ruthenium. 75% of the2-ethylanthraquinone were converted (selectivity: 100%).

Example 2

A stainless steel fabric (material number 1.4767) was heated in air at900° C. for 5 h in a muffle furnace. The fabric thus obtained was ringgear rolled and the corrugated piece of fabric was then rolled up with aflat piece of fabric. The monolith thus obtained was precisely fittedinto a continuous 0.3 l hydrogenation reactor.

2 g of ruthenium nitrosyl nitrate were dissolved in 500 ml of a 70:30Shell-sol® /tetrabutylurea mixture. This solution was continuouslymetered into the reactor in an amount of 60 ml/h at a hydrogen pressureof 10 bar and at 100° C. The reaction effluent obtained was colorlessand contained no ruthenium. After addition of the ruthenium-containingsolution was complete, the working solution (13% of 2-ethylanthraquinonein a 70:30 mixture of Shellsol®/tetrabutylurea) was continuously meteredinto the reactor in an amount of 300 ml/h at a hydrogen pressure of 10bar and at 40° C. without addition of ruthenium.

The conversion was 62% and the selectivity was 100%, based on2-ethylanthrahydroquinone, as determined by gas chromatography.

We claim:
 1. A process for hydrogenating an anthraquinone compound or amixture of anthraquinone compounds by contacting the anthraquinonecompound or mixture of anthraquinone compounds with a catalyst and purehydrogen or a hydrogen containing gas thereby preparing ananthrahydroquinone compound or a mixture of anthrahydroquinonecompounds, which comprises: effecting said contact with a catalystcomprising at least one homogeneous compound of at least one metal oftransition metal Group VIII of the Periodic Table of the Elementsdeposited alone or together with at least one metal of transition GroupI or VII of the Periodic Table of the Elements on a support in situ. 2.A process for hydrogenating an anthraquinone compound or a mixture ofanthraquinone compounds by contacting the anthraquinone compound ormixture of anthraquinone compounds with a catalyst and pure hydrogen ora hydrogen containing gas thereby preparing an anthrahydroquinonecompound or a mixture of anthrahydroquinone compounds, which comprises:effecting said contact with a catalyst comprising, as active metal, atleast one metal of transition Group VIII of the Periodic Table of theElements, except palladium, alone or together with at least one metal oftransition Group I or VII of the Elements, applied to a support, thesupport having a mean pore diameter of at least 50 nm and a BET surfacearea of at most 30 m²/g and the amount of active metal being from 0.01to 30% by weight, based on the total weight of the catalyst.
 3. Aprocess for hydrogenating an anthraquinone compound or a mixture ofanthraquinone compounds by contacting the anthraquinone compound ormixture of anthraquinone compounds with a catalyst and pure hydrogen ora hydrogen containing gas thereby preparing an anthrahydroquinonecompound or a mixture of anthrahydroquinone compounds, which comprises:effecting said contact with a catalyst comprising, as active metal,ruthenium, alone or together with at least one metal of transition GroupI or VII of the Elements, in an amount ranging from 0.01 to 30% byweight, based on the total weight of the catalyst, applied to a support,from 10 to 50% of the pore volume of the support being macropores havinga pore diameter ranging from 50 nm to 10,000 nm and from 50 to 90% ofthe pore volume of the support being mesopores having a pore diameterranging from 2 to 50 nm, the sum of the pore volumes being 100%.
 4. Aprocess for hydrogenating an anthraquinone compound or a mixture ofanthraquinone compounds by contacting the anthraquinone compound ormixture of anthraquinone compounds with a catalyst and pure hydrogen ora hydrogen containing gas thereby preparing an anthrahydroquinonecompound or a mixture of anthrahydroquinone compounds, which comprises:effecting said contact with a catalyst comprising, as active metal, atleast one metal of transition Group VIII of the Periodic Table of theElements, alone or together with at least one metal of transition GroupI or VII of the Periodic Table of the Elements, in an amount rangingfrom 0.01 to 30% by weight, based on the total weight of the catalyst,applied to a support, the support having a pore diameter at least 0.1 μmand a BET surface area of at most 15m²/g.
 5. A process for hydrogenatingan anthraquinone compound or a mixture of anthraquinone compounds bycontacting the anthraquinone compound or mixture of anthraquinonecompounds with a catalyst and pure hydrogen or a hydrogen containing gasthereby preparing an anthrahydroquinone compound or a mixture ofanthrahydroquinone compounds, which comprises: effecting said contactwith a catalyst supported on a monolithic support of a metal fabric or ametal foil prepared by sequentially heating said support material in airand then cooling the support material, followed by coating the supportmaterial with an active component under reduced pressure, andsubsequently cutting and shaping the coated support material and finallyprocessing to give a monolithic supported catalyst, whose active metalis at least one metal of transition Group VIII of the Periodic Table ofthe Elements, alone or together with at least one metal of transitionGroup I or VII of the Periodic Table of the Elements.
 6. A process forpreparing hydrogen peroxide by the anthraquinone process, whichcomprises: (1) hydrogenating an anthraquinone compound or a mixture ofanthraquinone compounds by a process as claimed in any one of claims 1,2, 3, 4 and 5 thereby preparing an anthrahydroquinone compound or amixture of anthrahydroquinone compounds; and (2) reacting theanthrahydroquinone compound or said mixture of anthrahydroquinonecompounds with an oxygen-containing gas to give a mixture comprisinghydrogen peroxide and the anthraquinone compound or said mixture ofanthraquinone compounds.
 7. A process as claimed in claim 6, wherein thesteps (1) and (2) are carried out continuously.
 8. A process as claimedin claim 6, which further comprises (3) extracting the hydrogen peroxidefrom the mixture comprising hydrogen peroxide and the anthraquinonecompound or said mixture of anthraquinone compounds with an aqueousextractant.
 9. A process as claimed in claim 7, which further comprises:(3) extracting the hydrogen peroxide from the mixture comprisinghydrogen peroxide and the anthraquinone compound or said mixture ofanthraquinone compounds with an aqueous extractant.
 10. The process asclaimed in claim 1, wherein a 2-alkylanthraquinone or a mixture of2-alkylanthraquinone compounds is the anthraquinone starting material.11. The process as claimed in claim 2, wherein a 2-alkylanthraquinone ora mixture of 2-alkylanthraquinone compounds is the anthraquinonestarting material.
 12. The process as claimed in claim 3, wherein a2-alkylanthraquinone or a mixture of 2-alkylanthraquinone compounds isthe anthraquinone starting material.
 13. The process as claimed in claim4, wherein a 2-alkylanthraquinone or a mixture of 2-alkylanthraquinonecompounds is the anthraquinone starting material.
 14. The process asclaimed in claim 5, wherein a 2-alkylanthraquinone or a mixture of2-alkylanthraquinone compounds is the anthraquinone starting material.15. The process as claimed in claim 6, wherein a 2-alkylanthraquinone ora mixture of 2-alkylanthraquinone compounds is the anthraquinonestarting material.
 16. The process as claimed in claim 3, wherein from10 to 45% of the pore volume of the support exists as macropores andfrom 55 to 90% of the pore volume exists as mesopores.
 17. The processas claimed in claim 16, wherein from 10 to 30% of the pore volume of thesupport exists as macropores and from 70 to 90% of the pore volumeexists as mesopores.
 18. The process as claimed in claim 16, whereinfrom 15 to 25% of the pore volume of the support exists as macroporesand from 75 to 85% of the pore volume exists as mesopores.